The present invention relates to a communication system including antenna selection diversity.
The Institute of Electrical and Electronic Engineers (IEEE), DRAFT SUPPLEMENT TO STANDARD FOR INFORMATION TECHNOLOGY-TELECOMMUNICATIONS AND INFORMATION EXCHANGE BETWEEN SYSTEMS—LOCAL AND METROPOLITAN AREA NETWORKS—SPECIFIC REQUIREMENTS—PART 11: WIRELESS LAN MEDIUM ACCESS CONTROL (MAC) AND PHYSICAL LAYER (PHY) SPECIFICATIONS: HIGH SPEED PHYSICAL LAYER IN THE 5 GHz BAND, IEEE P802.11A/D7.0, July 1999, is part of a family of standards for wireless Local and Metropolitan Area Networks (hereinafter LAN). The proposed standard specifies certain characteristics of a high speed, digital, wireless communication LAN based on Orthogonal Frequency Division Multiplexing (OFDM) and packet switching, incorporated by reference herein.
In an IEEE 802.11A network, data is transferred in data units that include a header and a data section. The data unit may be any general data structure, sometimes referred to as a packet or frame. The header of each data unit includes a preamble or OFDM training structure comprising a “short training sequence” followed by a “long training sequence.” The “long training sequence” comprises two 3.2 μs duration symbols. It is to be understood that symbols may be any type of signal, different durations, different amplitudes, different frequencies, and different characteristics, as desired. The long training sequence is used for channel and fine frequency offset estimation. The short training sequence comprises ten repetitions of a 0.8 μs duration symbol for a total sequence length of 8 μs. During the short training sequence the receiver normally performs signal detection, automatic gain control (AGC), coarse frequency offset determination (CFOD), and timing synchronization. In addition, the receiving device may perform energy determination and antenna diversity selection.
A detection circuit of a receiving device converts an analog radio frequency (R/F) signal received at the antenna to a digital signal and determines whether the received signal is sufficiently strong to be recognizable above the noise in the communication system. The signal detection circuit senses the presence of a signal. The strength of the received R/F signal can vary by orders of magnitude. On the other hand, the analog-to-digital (A/D) signal converter of the detector requires a relatively constant amplitude input signal to avoid clipping and loss of message bits. Typically, an automatic gain control (AGC) circuit controls the amplitude variation of the R/F signal at the input to the A/D converter while the amplitude of the received R/F signal is varying.
The transmitting device and the receiving device each include a clock circuit, normally implemented as an oscillator. In order to synchronize the frequency relationship of the transmitted and received signals, the CFOD circuit synchronizes the frequency of the oscillator in the receiving device to match that of the received signals. In this manner the receiving device adjusts the oscillator to match the actual frequencies of the received symbols. In order to synchronize the temporal relationship of the transmitted and received signals, the timing synchronization circuit synchronizes the temporal relationship of the oscillator in the receiver to match that of the received signals. In this manner the receiving device determines where each symbol actually starts.
With high transmission frequencies, such as in the range of 5-6 GHz, the resulting wavelength of the signal is on the order of five centimeters. With such a short wavelength the receiving device, such as a wireless telephone, may be periodically located in an unsuitable phase relationship with respect to the received signal. In other words, the receiving device may be positioned at a location where the signal is at a minimum making reception difficult, if at all possible. Accordingly, it is preferable to include multiple antennas interconnected to the same receiving device at spaced apart locations. With multiple spaced apart antennas it is likely that at least one antenna will sense a strong signal. Normally the antenna sensing the strongest signal is selected to receive the following data unit.
Referring to FIG. 1, one possible receiving device 20 includes a pair of spaced apart antennas 22a and 22b. Each of the antennas 22a and 22b is interconnected to a respective detection circuit 24a and 24b. Each of the detection circuits 24a and 24b performs signal detection, automatic gain control (AGC), energy determination, coarse frequency offset determination (CFOD), and timing synchronization. The energy determination determines which antenna 22a, 22b senses the strongest signal, normally using a correlator, and accordingly a switch circuit 26 selects the antenna 22a, 22b with the strongest signal to receive the following data unit. Unfortunately, including a pair of detection circuits 24a, 24b within the receiving device 20 consumes twice the power of a single detection circuit and increases the expense of the receiving device 20.
What is desired, therefore, is a receiving device that includes antenna diversity with a single detection circuit, especially a receiving device suitable for a P802.11A.